The present invention relates to biodegradable thermoplastic composition suitable, in particular for preparing injection moulded articles free from surface defects, comprising a starchy component and a cellulose ester or ether.
Biodegradable composition obtained starting from starch and a thermoplastic polymer are well known in the art and available from the market and are disclosed, e. g., in European patent applications EPA 32 802, 327 505, 400 532, 404 723, 404 727, 404 728, and in W090/0671, W071/02025, U.S. Pat. No. 5,095,054.
Typically, such compositions can be obtained by blending starch and a synthetic thermoplastic polymer, under extrusion cooking conditions, i.e. in the presence of limited amounts: of water (generally 0.5-40% by weight based on starch-water system) or of a plasticiser, by operating under temperature and pressure conditions sufficient to destroy the starch crystallinity and to obtain a thermoplastic molten mass (destructured starch).
From European patent application EPA 575 349, biodegradable compositions are known which comprise a starch component and a cellulose ester.
The injection moulded articles obtained starting from said compositions are affected from the drawback of displaying unpleasant surface scale due to the poor compatibility between starch and cellulose esters. European patent application EPA 542 155 discloses compositions based on starch and cellulose esters added with a compatibilizer agent (epoxidized soy bean oil and acetylated starch) in order to confer improved mechanical properties to the moulded articles. The moulded articles from the above compositions undergo considerable delamination phenomena due to the insufficient compatibilization between starch and the cellulose ester.
We have found now that it is possible to improve the mutual compatibility of starch or starch esters or ethers and cellulose esters or ethers and to obtain moulded articles free from scales unevenness, by using selected classes of compatibilizing agents.
Beside endowing the resulting compositions with better homogeneity, the use of the compatibilizing agents of the invention improves the biodegradability characteristics of the compositions.
The compositions according to the present invention comprise:
starch or a starch ester or ether with a substitution degree from about 1.2 to 2.5;
a cellulose ester or ether with a substitution degree from about 1.2 to 2.5;
a plasticizer for the starchy phase and a plasticizer for the cellulose derivatives phase or a plasticizer for both phases;
a compatibilizing agent selected from the following classes:
(A) polymers compatible with cellulose esters or ethers and/or starch or starch esters and ethers, grafted with aliphatic or polyhydroxylated chains containing from 4 to 40 carbon atoms;
(B) copolymers obtained from hydroxy acids and/or diamines with 2-24 carbon atoms and aliphatic or aromatic diisocyanates or epoxy compounds and anhydrides; copolymers obtained from aliphatic polyesters, polyamides or polyureas and aliphatic or aromatic diisocyanates; copolymers obtained from aliphatic or aromatic diisocyanates and polyalkylene glycols;
(C) copolymers obtained from polymers compatible with cellulose esters or ethers and/or starch or starch esters or ethers, by grafting starch soluble polyols or structures capable of complexing starch.
(D) polymers capable of complexing starch such as ethylenexe2x80x94vinyl alcohol or ethylene acrylic acid copolymers, aliphatic polyesters or polyamides.
(E) starch compatible polyols selected from the monomers and the low molecular weight polyols (Viscosity average molecular weight lower than 10,000) such as glycerol, sorbitol, erythritol, polyglycerol, dextrines, polyvinylalcohol, polyaspartates, and the above polyols grafted with alkylenoxides or polyalkylenoxides. The compatibilizer agents indicated under (A) are obtained by grafting aliphatic chains with 4-40 carbon atoms optionally containing up to three unsaturations and or heteroatoms or still other functional groups, to polymers compatible with cellulose esters or cellulose ethers. Preferably, the chains derive from animal or vegetable fats, such as oleic, lauric, myristic, palmitic, stearic, euric, linoleic, ricinoleic acids or phospholipids with terminal end groups which allow the chains to be grafted to the polymers compatible with cellulose esters or ethers.
The terminal group can be carboxy, ester or salt groups; the chains can also be modified in order to obtain terminal groups such as alcohol, aldehyde, amine, amide, acid chloride, isocyanate, mercaptan epoxy and anhydride groups. The polymers to which the above said lipidic chains are grafted can display different degrees of affinity with cellulose derivatives and starch esters or ether; some of them can even be miscible with said cellulose derivatives and starch ester or ethers; others, with a lower compatibility degree, can anyway result to be interesting because they can be easily transformed into compatible derivatives.
Such polymers can be of either natural or synthetic origin. Furthermore, they can be used as such, or modified or depolymerized to trimer level by hydrolysis, saponification, cracking or by means of enzymatic reactions.
Examples of the above polymers are:
(a) cellulose esters with various DS (Degree of Substitution);
(b) cellulose ethers with various DS;
(c) cellulose ethers/esters with various DS values;
(d) starch ester with various DS values, as acetates;
(e) starch ethers with various DS values, such as the reaction products of starch with ethylene or propylene glycols;
(f) starch ethers/esters with various DS values;
(g) partially hydrolysed polyvinyl acetate;
(h) aliphatic polyesters and copolyesters, optionally also grafted with those products as listed under (a)-(g) above. In this case, polymers are preferred which are obtained by grafting low molecular weight(350-1000) polycaprolactone (PCL) to polyvinyl alcohol copolymer or, also, by grafting PCL to regenerated cellulose or starch;
(i) aliphatic/aromatic or aromatic copolyesters optionally grafted with above (a)-(g) products;
(j) polymers from natural origin such as cellulose, hemicellulose, lignin, cellulose ethers and xanthates, regenerate cellulose, pullulan, chitin, chitosan,pectins, proteins, vegetable and animal gelatines, zein, gluten, casein, albumen, natural or modified rubbers, alginates, rosin derivatives.
The aliphatic chains can be grafted by means of any known type of reaction, and generally by:
(1) transesterification of ester group;
(2) esterification of hydroxy groups;
(3) urethanizing hydroxy groups by means of isocyanates;
(4) epoxidizing hydroxy groups with aliphatic epoxides;
(5) acetilization of hydroxy groups with aliphatic aldehydes.
The compatible polymers with the cellulose derivatives and starch esters or ethers, polyols soluble in starch, or capable of complexing starch, can be grafted for example with following polyols: modified amylose and its hydrolysis product; polyvinyl alcohol with various hydrolysis degrees, ethylene-vinyl alcohol copolymers, polyols of glycerol, polyglycerol, saccharides, oligosaccharides, trimethylol propane, pentaerythritol.
The number of grafted chains are comprised within the range of from 0.1 to 30 grafted chains per each 100 monomeric units in the polymer chain, preferably from 0.2 to 20, and still more preferably, from 0.3 to 10 grafted chains for each 100 monomeric units.
Besides the compatibilizers of above (A) type which require that polymeric products are modified by grafting lipidic chains, also copolymers of above (B) type can be advantageously used, particularly those obtained from such aliphatic polyesters such as polycaprolactone with various molecular weight and polyethylene succinates, from alipathic or aromatic diisocyanates or copolymers obtained from C2-C24 hydroxy acids, or aliphatic or aromatic diisocyanates, or copolymers of above (C) type.
For the preparation of copolymers of above (B) type, preferred diisocyanates are:
4,4xe2x80x2-diphenylmethane diisocyanate, hydrogenated 4,4xe2x80x2-diphenylmethane diisocyanate, toluidene diisocyanate, isoforone or hexamethylene-diisocyanate.
Estane (a caprolactone:urethane copolymer traded by Goodrich as xe2x80x9c54351 gradexe2x80x9d) is a representative copolymer of (B) class.
The compositions comprise the starchy component and the cellulose derivatives in ratios by weight comprised within the range from 1:90 to 90:1, preferably from 1:40 to 40:1 and still more preferably of 1.5:5 to 1:1.5.
The compatibilizer agents are present in amounts comprised from 0.1 to 20% by weight, preferably of from 0.5 to 10%
The plasticiser for cellulose phase and the starchy phase are present in amounts respectively comprised from 5 to 40%, and still more preferably,of from 10 to 30%, by weight.
Compounds which act as plasticizers for both phases, such as, diacetins, can be used.
The total amount of plasticizer is generally comprised from 5 to 40% based on total weight of the composition, preferably from 10 to 30%.
Besides the components indicated above, the composition can also contain synthetic polymers in an amount up to about 30% by weight, preferably less than 10%.
Examples of synthetic polymers which can be used are polyvinylalcohol, polyvinyl acetate, thermoplastic polyesters, such as polycaprolactone, copolymers of caprolactone with isocyanates, polyethylene or polybutylene adipate or sebacate, lactic acid polymers.
The starch which can be used to prepare the compositions according to the present invention generally is a native starch, extracted from various plants, such as maize, potato wheat, tapioca and cereal starch. Under the term xe2x80x9cstarchxe2x80x9d are also included high amylopectin starches (xe2x80x9cwaxyxe2x80x9d starches), high-amylose starches, chemically and physically modified starches e.g., starches the acid number of which is reduced down to a value comprised within the range from 3 to 6; starches in which the type and concentration of cations associated with phosphate groups are modified, starch ethoxylated,starch acetates, cationic starches, hydrolysed starches, oxidized and crosslinked starches.
Representative cellulose esters comprise cellulose acetates, propionates and/or butyrates, with various degrees of substitution. Cellulose acetate with degree of substitution comprised from 1.5 to 2.5 are preferred.
Example of cellulose and starch ethers are ethyl or propyl ethers.
Cellulose esters or ethers in neat form, mainly the acetate esters, have so high processing temperature as to cause the matrix to undergo severe degradation. They require the use of a plasticizer which can be selected from:
glycerol esters with aliphatic acids containing up to 6 carbon atoms, in particular diacetin and triacetin;
esters of citric acid, in particular trimethyl or triethyl citrate, as well as acetyl-triethyl-citrate;
dialkyl esters of tartaric acid;
esters of aliphatic acids, lactones and lactides;
dialkyl esters of aliphatic acids such as those derived from oxalic, glutaric, adipic, sebacic, suberic, azelaic acids, mainly dibutyl adipate and dibutyl sebacate;
dialkyl esters of aromatic acids in which the alkyl group contains from 1 to 10 carbon atoms, in particular dimethyl phthalate, diethyl phthalate, methoxy and ethoxy ethyl phthalate;
polyethylene glycol adipate, glutarate or sebacate;
alkyl and aryl phosphates, in particular triethyl and tricresyl phosphates;
alkyl ester or fatty acids, such as butyl oleate;
plasticizers such as the product traded as Paraplex an epoxidized soybean oil available from Rohm and Haas; Admex 719 an epoxidized talloil available from Archer Daniels Midland; the Flexol series of 2-ethylhexyl esters of hexanediole acids from Union Carbide;
non-bleeding plasticizers such as: mixed aliphatic-aromatic esters of trimethlyol propane and pentaerythritol; alkyl phosphate terminated polyethylene glycols.
In order to obtain the thermoplastic character of the starchy phase, in particular at low humidity contents, polar substances are added, which are capable of forming hydrogen bonds with amylose and amylopectin. Suitable substances for that purpose are polyols with 1-20 repeating hydroxylated units, each containing from 2 to 6 carbon atoms; ethers, thioethers, organic and inorganic esters, acetates and amino derivatives of the above polyols; the reaction products of the polyols with chain extenders; polyol oxidation products containing at least one aldeydic or carboxy group. Plasticizers of this type are disclosed in WO 92/19680 application.
The plasticizers disclosed in WO 92/19680 include
The plasticizers incorporated from WO 92/19680 include glycerine, polyglycerol, glycerol ethoxylate, ethylene or propylene glycol, ethylene or propylene diglycol, ethylene or propylene triglycol, polyethylene or polypropylene glycol, 1,2-propandiol, 1,3-propandiol, 1,2-butandiol, 1,3-butandiol, 1,4-butandiol, 1,5-pentandiol, 1,6-pentandiol, 1,5-hexandiol, 1,2,6-hexantriol, 1,3,5-hexantriol, neopentylglycol, trimethylolpropane, pentaerythritol, sorbitol acetate, sorbitol diacetate, sorbitol monoethoxylate, sorbitol dipropoxylate, sorbitol diethoxylate, sorbitol hexaethoxylate, aminosorbitol, trihydroxymethylaminomethane, glucose/PEG, the product of reaction of ethylene oxide with glucose, trimethylolpropane monoethoxylate, mannitol monoacetate, mannitol monoethoxylate, butyl glucoside, glucose monoethoxylate, alpha-methyl glucoside, the sodium salt of carboxymethylsorbitol, polyglycerol monoethoxylate and mixtures thereof.
Preferred plasticizers are those which act as plasticizers for both the starchy and the cellulosic phase and the acetins belong to this preferred class.
The starchy phase is normally plasticised directly during the compounding step, in mixture with all the other components. However, also a two step process can be used in which the starchy phase and the cellulosic phase are individually plasticized and/or the starchy phase is plasticized prior to the final compounding step.
Preferred plasticizers comprise: water glycerol, glycerol ethoxylate, ethylene or propyleneglycol, polyethylene glycol polypropyleneglycol, 1,2-propanediol, 1-3 propanediol, 1-2, 1,3-, 1,4-butanediol, 1,5-pentanediol, 1,6-, 1,5-hexanediol, 1,2,6, 1,3,5,-hexanetriol, neopentylglycol, sorbitol acetate, sorbitol diacetate, sorbitol monoethoxylate, sorbitol diethoxylate, trimethol propane monoethoxylate, mannitol monoacetate, mannitol monoethoxylate, butyl glucoside, glucose monoethoxylate, sodium salt of carboxymethylsorbitol, polyglycerol monoethoxylate.
The preparation of the composition according to the present invention comprises blending the components in a heated extruder or in any device which may secure temperature shear stress conditions sufficient to cause the conversion of the starchy material and the cellulose derivatives into a thermoplastic state and to render the components compatible with each other as regard their rheology, operating at a temperature comprised within the range of from 80 to 210xc2x0 C. in the presence of water and a plasticizer.
The preferred method for preparing the compositions comprises:
a first step consisting in conveying the components through an extruder with residence times of the order of from 2 to 50 seconds, during which the starchy components and cellulose derivatives are caused to undergo swelling due to the action of the plasticizer and optionally added water, by operating at a temperature comprised within the range from 80 to 180xc2x0 C.;
a blending step during which the mixture from the preceding step is submitted to shear stress conditions which correspond to similar viscosity values of the cellulose derivatives and the starchy components;
optionally a step of venting under controlled pressure conditions or in vacuo in order to obtain a molten mass preferably at a temperature comprised within the range from 130 to 180xc2x0 C. with a water content preferably smaller than 6% such as not to have bubbles formation at atmospheric pressure, e.g., at the extruder outlet, if producing foamed products is not desired. This condition is satisfied when it is desired to obtain not foamable products.
In the case of foamable products, the water content in the blend may be as high as 20%, preferably up to 18%.
The resulting molten mass can be then directly extruded into pellets from which articles are then fabricated by means of the conventional processes.
The following examples are supplied in order to illustrate the invention without limiting it.